The present invention generally relates to an arrangement for, and a method of, examining gemstones, especially cut diamonds, by illuminating the gemstone, capturing images of the illuminated gemstone, and analyzing the captured images with repeatability.
The beauty and price of a gemstone, such as a cut multi-faceted diamond, are based, for example, on its cut, carat weight, clarity and color. Many different geometrical patterns of cuts, such as round brilliant, oval, pear, marquise, radiant, princess, heart, emerald, etc. are now standardized. The cut, the carat weight, the clarity and the color of the gemstone are typically evaluated and/or measured by a human appraiser. Such evaluated and/or measured properties are often objectively presented to a consumer, typically in certificate form, for price valuation.
Optical performance of the gemstone, that is, how the gemstone “plays with light”, as well as optical efficiency, that is, how the gemstone “reflects light”, are difficult to subjectively evaluate and measure, even for the experienced human appraiser. Optical performance and efficiency of the gemstone are typically characterized by such properties as its brilliance (the amount and intensity of incident light returned from the gemstone), scintillation (fast and local fluctuations in the incident light returned as the gemstone moves), fire (the dispersion of incident white light into its spectral colors), coverage (the area of the incident light returned compared to the total area of the gemstone table), contrast (the intensity of the incident white light returned compared to the intensity of the non-returned or black light), and symmetry (the balance of the pattern of the incident light returned). A more visually active gemstone is deemed more valuable than a less visually active gemstone, even with the same cut, carat weight, clarity and color.
To objectively measure such optical properties of gemstones, the art has disclosed various computer-based systems for capturing and analyzing images of gemstones illuminated under varying lighting conditions. For example, U.S. Pat. No. 5,615,005 discloses a gemstone evaluation system that captures images of a gemstone placed table-side face-down on a glass plate in an analysis chamber and illuminated from a plurality of different angles by a movable light source that is moved toward the gemstone during the evaluation. Captured images of the gemstone are analyzed by a computer, and various optical properties of the gemstone are measured and displayed or printed.
As another example, U.S. Pat. No. 6,813,007 discloses another computer-based system that captures images of a gemstone also placed table-side face-down on a glass plate, but illuminated by light reflected off a rotary reflector that rotates during the evaluation. Captured images of the gemstone are again analyzed by a computer, and various optical properties of the gemstone are measured and displayed or printed.
However, as advantageous as such computer-based systems have been, they have not proven to be altogether satisfactory in use. A moving system component, such as a rotating reflector or a moving light source, causes mechanical variability and vibrations that can interfere with the measurements, and degrade measurement accuracy and repeatability. The light source also concomitantly generates heat that can interfere with the measurements. Also, the glass plate on which the gemstone is placed creates an interface at which light interference can occur due to light refraction, thereby again degrading measurement accuracy and repeatability. This interference effect is aggravated by frequent contamination with dust, dirt, oil from an operator's fingers, scratches, or like contaminants on the glass plate. Furthermore, the placement of the gemstone on the glass plate is variable, thereby still further worsening measurement accuracy and repeatability. An objective, accurate and repeatable examination of a gemstone is essential for true price valuation of the gemstone.